1. Field of the Invention
The present invention relates to a image forming apparatus in an electrostatic transfer system. More particularly, the present invention relates to cleaning of a transfer device for transferring a toner image of a photosensitive body to a transferred material.
2. Description of the Related Art
In a transfer device in a general image forming apparatus, a bias having polarity reverse to that of toner is applied to a transfer roller so that a toner image on a photosensitive body is transferred to a transferred material such as paper.
In this case, a rear side of the transferred material becomes dirty if no attachment of toner onto the transfer roller is prevented at a pre-rotating time of the transfer roller in an initial transfer operation and an interval of sheets of paper at a continuous paper feeding time.
Therefore, Japanese Patent Application Laying Open (KOKAI) No. 3-69978 shows a technical structure for preventing the attachment of toner onto a transfer member. In this structure, a bias is applied to the transfer member when there is no transferred material. This bias has a predetermined polarity and is applied to the transfer member for a time equal to or greater than a time required to rotate the transfer member at one time. Thereafter, the polarity of the bias is reversed and this bias is applied to the transfer member for a time equal to or greater than a time required to further rotate the transfer member at one time, thereby preventing toner from being attached to the transfer member.
However, when the above bias is switched to a bias at a transfer time at fast timing corresponding to one rotation of the transfer member, normal toner is developed since a voltage such as several ten volts is applied to the photosensitive body and this photosensitive body has polarity reverse to that of the normal toner. Therefore, a rear side of the transfer member becomes dirty at the transfer time.
There is also a problem that a power source is burdened when polarities of the bias are rapidly changed from a cleaning time to the transfer time, and from the transfer time to the cleaning time.